Preparation Of BN Nanomaterial S Via A Soft Template Method

Based on the three-step synthesis, the precursor is first prepared in 1,4-dioxane liquid system with sodium borohydride(Na BH4) as source of boron and ammonium fluoroborate(NH4BF4) as nitrogen source, which is ammonia borane clear solution. Then, the mixed solvent composed by a certain amount of the solution and soft template agent(PEG2000, PEG4000, PEG20000, octadecylamine, SDS) solution is poured into a autoclave which is encapsulated and heated to 160℃, and then the white powder is obtained by drying the mixed solvent during subsequent heat treatment by the rotary evaporation apparatus. In the end, the white powder is heated to several different temperatures(700℃, 800℃, 900℃, 1000℃) under ammonia atmosphere in the tube furnace and 700 ℃under the air in the muffle furnace, so the obtained final product is boron nitride(BN) samples. XRD, FE-SEM, FTIR and PL were used to characterize the products. In this paper, the preparation of boron nitride nanomaterials is divided into three parts: solvent heat treatment, soft template agent assisted synthesis, ammonia heat treatment. Then the mechanism of soft template agent was discussed.Firstly, during the solvothermal stage, amorphous hybrid substance is obtained by non-covalently combination between soft template agent(polyethylene glycol(PEG), octadecylamine and SDS) and precursor in the liquid phase system(1,4-dioxane as solvent).During ammonia treatment stage, because of enough heat energy and the role of the soft template agent(PEG, octadecylamine and SDS) inducting hexagonal boron nitride nucleation and growth, hybrid substance is promoted the crystallization process; at the same time, C, O-doped BN nanomaterials containing the C-N and N-B-O bonds is generated due to part C and O atoms of the soft template agent combination on the BN skeleton in the form of a covalent bond. After subsequent air burning stage, B-N and B-N-B bonds vibration intensity increase, which shows that polymorphs is more complete and the crystallization degree of samples increases. When different molecular weight PEG is used as the soft template agent, C-N and C=C bonds vibration intensity of the final samples gradually increase with the increase of PEG molecular weight, which is caused an increase in C defect impurity concentration, so it results in the emission peaks redshift. Compared with the sample of octadecylamine as the template agent, the sample emission peaks of the SDS as soft template agent have a certain redshift. With the increase of the ammonia heat treatment temperature, the crystallization degree of BN improves. When ammonia heat treatment temperature is 1000℃, the crystallization degree of h-BN nanomaterials is very high in the supporting role of soft template agent, which is far higher than boron nitride without template agent assisted. The crystallization degree of samples is affected by solvothermal process, ammonia heat treatment temperature, the type and adding amount of soft template agent. From the perspective of thermodynamics, it is successfully explained BN nucleation and growth mechanism by the Gibbs free energy of nucleation theory, which is promoted by soft template agent.